The vast majority of animal species reproduce sexually, combining genetic material from two parents to produce offspring that are genetically distinct from either one. From corals releasing clouds of eggs and sperm into open water to octopuses using a chemically guided arm to deliver sperm directly to a mate’s oviduct, the basic principle is the same: two specialized cells, called gametes, fuse to create a new organism. The process looks wildly different depending on the animal, though, and the reasons sex persists at all, despite its steep biological costs, remain one of the more fascinating puzzles in biology.
Why Sex Exists When Cloning Would Be Easier
Sexual reproduction is expensive. An animal that reproduces asexually passes all of its genes to every offspring. A sexual animal passes only half. On paper, cloning should outcompete sex within a few generations. Yet sex is nearly universal among animals. The persistence of sex has generated decades of competing theories, and current evidence points to two main explanations working in tandem.
The first is that sex speeds up adaptation. When beneficial mutations arise in different individuals, clonal populations can only adopt them one at a time, because each mutation is stuck in its own genetic lineage. Sex shuffles those mutations together, letting a population accumulate advantages faster. Modeling work has shown that sexual populations outcompete clonal ones partly because clonal lineages interfere with each other’s selection: many similarly adapted clones compete, slowing down the rate at which any one beneficial change spreads. Sex eliminates that interference and accelerates adaptation.1PubMed Central. The maintenance of sex: Ronald Fisher meets the Red Queen
The second explanation involves parasites. The Red Queen hypothesis proposes that hosts need to constantly reshuffle their genotypes to stay ahead of parasites that evolve to exploit the most common host genotype. A clone army is a sitting target; a sexually reproducing population is a moving one. Reviews of both theoretical and experimental evidence suggest these two ideas, faster spread of beneficial mutations and defense against parasites, are the leading explanations for why sex has persisted across the animal kingdom.2PubMed. Current hypotheses for the evolution of sex and recombination
Eggs, Sperm, and Why They Are Different Sizes
Sexual reproduction hinges on two types of gamete coming together. In almost all sexually reproducing multicellular animals, one sex produces large, nutrient-rich gametes (eggs) and the other produces small, mobile gametes (sperm). This size difference between gametes, called anisogamy, is itself the original sexual dimorphism and the foundation on which male and female roles are built.
The prevailing explanation for why gametes diverged in size involves two opposing pressures. Making many small gametes increases the odds that at least one finds a partner. Making fewer, larger gametes gives the resulting embryo more nutrients and a better survival chance. When a larger zygote provides a disproportionate boost to offspring survival, selection pushes gamete sizes apart: one type gets smaller and more numerous, the other gets bigger and better provisioned.3PubMed Central. A comparative test of a theory for the evolution of anisogamy This divergence sets up the basic dynamic of sexual reproduction across most animal groups, from sea urchins to elephants.
How anisogamy shapes competition between mates has been debated. The common assumption is that because sperm are cheap and abundant, males should always compete more intensely for mating opportunities. But recent modeling suggests this depends heavily on the conditions under which anisogamy originally evolved, whether gametes were competing to find each other in open water or were limited by scarcity of encounters.4bioRxiv. The evolution of anisogamy does not always lead to male competition The picture is more nuanced than the textbook version implies.
External Versus Internal Fertilization
Animals have solved the problem of getting egg and sperm together in two fundamentally different ways. In external fertilization, both gametes are released into the surrounding environment, usually water, and fusion happens outside the body. Most fish, many amphibians, corals, sea urchins, and countless marine invertebrates reproduce this way. In internal fertilization, sperm are delivered directly into the female’s reproductive tract, and the egg is fertilized inside her body. All mammals, birds, reptiles, and most terrestrial invertebrates use internal fertilization.
The evolutionary trajectory has been overwhelmingly one-directional. Across vertebrates, researchers have identified roughly thirteen independent transitions from external to internal fertilization and no confirmed reversals.5PubMed Central. Fertilization mode differentially impacts the evolution of vertebrate sperm components Once a lineage moves to internal fertilization, it stays there. This makes sense: internal fertilization gives the female far more control over which sperm reaches her eggs and protects embryos from environmental hazards, advantages that are hard to give up.
Even among external fertilizers, gamete fusion is not random. In broadcast spawning marine invertebrates, sperm actively follow chemical gradients released by eggs, swimming preferentially toward eggs whose chemical signals predict healthier offspring. Experiments have shown that sperm show consistent preferences for certain females’ eggs, and those preferences predict which pairings produce the most viable embryos.6PubMed Central. Chemically moderated gamete preferences predict offspring fitness in a broadcast spawning invertebrate Fertilization in open water, in other words, is a lot less random than it looks.
Internal fertilization has evolved independently in surprising places. Among killifish alone, the transition happened at least three separate times in different subfamilies, each time accompanied by the independent evolution of modified fins that function as copulatory organs.7PubMed. The evolution of copulatory organs, internal fertilization, placentae and viviparity in killifishes (Cyprinodontiformes) inferred from a DNA phylogeny of the tyrosine kinase gene X-src The repeated, independent invention of the same reproductive strategy in unrelated groups is strong evidence that internal fertilization provides powerful advantages, especially in unpredictable or terrestrial environments.
How Sex Is Determined
Which sex an animal develops as is controlled by remarkably different mechanisms across the animal kingdom. Mammals use the familiar XX/XY system, where females carry two X chromosomes and males carry an X and a Y. Birds flip the script: males are ZZ and females are ZW. Some insects, like grasshoppers, have no Y chromosome at all; sex depends on whether an individual has one X or two. And in many reptiles, sex is determined not by chromosomes but by the temperature at which eggs are incubated. A nest of turtle eggs laid in cooler sand may hatch mostly males, while a warmer nest produces mostly females.
What is striking is how all of these different starting signals eventually funnel into broadly conserved downstream pathways that build male or female anatomy. The initial trigger, whether a gene on a chromosome or the temperature of the surrounding soil, activates regulatory networks that are far more similar across species than the triggers themselves. Evolution has been remarkably creative with the on-switch but surprisingly conservative with the machinery it turns on.
Animals That Change Sex
Some animals do not lock into one sex for life. Sequential hermaphroditism, where an individual functions as one sex early in life and switches to the other later, is widespread in fish, some invertebrates, and even certain plants. The shift can go in either direction: protandry means starting male and becoming female, while protogyny means starting female and becoming male. Clownfish are a well-known example of protandry; many wrasses and groupers are protogynous.
The logic behind sex change is straightforward: an individual switches when it will reproduce more successfully as the other sex. In species where a large male can monopolize mating access to many females, it pays to start life as a small female and switch to male only after growing large enough to compete. The reverse holds when large body size gives females a fecundity advantage. This is an evolutionarily stable strategy when fecundity rises faster with age in the second sex.8PubMed Central. Consequences of sex change for effective population size
Recent work has revealed that sex-changing animals use a wider range of strategies than biologists originally recognized. Some individuals change early, others late. Some change sex more than once. Others in the same species never change at all. The timing appears to be an adaptive response to each individual’s social and ecological circumstances, with variation in those conditions producing significant differences in when or whether sex change occurs.9Trends in Ecology & Evolution. Why and when sex change occurs
When Sexual Species Reproduce Without Mating
Some animals that normally reproduce sexually can occasionally skip the process entirely. Facultative parthenogenesis, where a female produces viable offspring from an unfertilized egg, has been documented in species you might not expect: sharks, sawfish, snakes, Komodo dragons, and various insects. These are not species that have given up on sex. They are sexual species with a backup option.
A hammerhead shark held in captivity without any males produced a pup through parthenogenesis, raising questions about how common this might be in the wild.10PubMed Central. Virgin birth in a hammerhead shark More remarkably, researchers studying wild smalltooth sawfish, a critically endangered species, found individuals that appeared to have been produced by parthenogenesis living normally alongside sexually produced fish. These parthenogenetic sawfish were normal-sized and survived for at least a year, demonstrating that “virgin births” can produce viable individuals in nature, not just in aquariums.11Current Biology. Facultative parthenogenesis in a critically endangered wild vertebrate
Other animals toggle between sexual and asexual reproduction as a routine part of their life cycle. Water fleas in the genus Daphnia reproduce asexually through parthenogenesis when conditions are favorable but switch to sexual reproduction when the environment becomes stressful. This cyclical parthenogenesis involves measurable changes in gene expression, with sexual females ramping up meiosis-related genes and asexual females increasing metabolic gene activity instead.12PubMed Central. The transcriptomic signature of cyclical parthenogenesis The switch is essentially a bet-hedging strategy: clone yourself when times are good, shuffle the genetic deck when times get tough.
Then there are the bdelloid rotifers, microscopic animals that appear to have abandoned males entirely for tens of millions of years. Despite this, their genomes show signatures that look suspiciously like the aftermath of recombination. Recent genomic work suggests they may be performing a kind of abortive meiosis, pairing chromosomes without completing a proper sexual cycle, which could give them some of the genetic reshuffling benefits of sex without actually needing a mate.13Trends in Genetics. Genomics and genetics of bdelloid rotifers
What Happens After Mating
In many species, mating is not the end of the story. Females that mate with multiple males can influence which sperm actually fertilizes their eggs, a phenomenon called cryptic female choice. This post-mating selection is widespread and can have profound consequences.
In chinook salmon, females that mated with related males released fewer sperm-binding chemicals on their eggs than when mating with unrelated males. Closer investigation showed that the critical variable was not overall genetic relatedness but similarity at a specific immune gene complex. Females preferentially allocated fertilization to males whose immune genes were most different from their own, which would give offspring a broader immune repertoire.14PubMed Central. Cryptic female choice favours sperm from major histocompatibility complex-dissimilar males
Cryptic female choice can even influence whether closely related species stay separate. Conspecific sperm precedence, where a female’s reproductive tract biases fertilization toward males of her own species, has been documented across many animal groups. Modeling work suggests that this mechanism alone can maintain reproductive isolation between species under realistic conditions, particularly when migration between populations is low and females mate with multiple males.15Evolution. Cryptic female choice can maintain reproductive isolation In other words, what happens inside the female’s body after mating can be just as important as mate choice itself for keeping species distinct.
Specialized Sperm Delivery in Cephalopods
Some of the most elaborate mating apparatus in the animal kingdom belongs to squid and octopuses. Male cephalopods typically use a modified arm, called the hectocotylus, to transfer packages of sperm called spermatophores to the female. The arm is not just a delivery tool; it is a sensory organ.
In octopuses, the hectocotylus uses contact-dependent chemical sensing to identify females and navigate their internal anatomy, guided by the hormone progesterone. The arm literally tastes its way to the oviduct.16PubMed Central. A sensory system for mating in octopus In certain squid species, the hectocotylus carries a sexually dimorphic glandular system with specialized secretory cells that may coat spermatophores in a viscous substance, help them adhere without damage, or even chemically influence the female’s behavior.17PubMed. Getting a grip on the squid hectocotylus: Sexually dimorphic glandular system underlies spermatophore transfer in Doryteuthis pleii (Blainville, 1823)
The Hormones That Run the Cycle
Sexual reproduction in animals is orchestrated by hormones, but the specific hormones vary across groups more than many people realize. In vertebrates, the familiar players are gonadotropins released by the brain’s pituitary gland, which stimulate the gonads to produce sex steroids like estrogens and androgens. These steroids drive everything from egg maturation and ovulation to sperm production and mating behavior.
Insects were long assumed to lack sex hormones entirely, but that turns out to be an oversimplification. Insects use juvenile hormone and ecdysteroids to control gonad development, and while their gonadotropins are structurally unrelated to vertebrate ones (none are glycoproteins, and they do not fall into a single peptide family), the functional parallels are real.18PubMed. Gonadotropins in insects: an overview Evidence has accumulated that ecdysone at high concentrations may function as an androgenic steroid in insects, and that certain steroidogenic transcription factors are remarkably similar between insects and mammals.19PubMed. “Insects do not have sex hormones”: a myth?
A unifying observation is that sharp spikes in steroid hormones serve as signals across very different animal groups. In vertebrates, progesterone and estradiol peaks drive the reproductive cycle. In insects, ecdysteroid surges accompany molting and metamorphosis. Both appear to be linked to programmed cell death in reproductive tissues, suggesting an ancient connection between steroid signaling, tissue remodeling, and reproduction.20PubMed. The endocrine system controlling sexual reproduction in animals: Part of the evolutionary ancient but well conserved immune system?
Pregnancy and the Immune Puzzle
For animals that carry developing young internally, sexual reproduction creates a profound immunological problem. The embryo carries genes from the father and is therefore genetically foreign to the mother. In any other context, the immune system would attack foreign tissue. Mammalian pregnancy requires the mother’s immune system to tolerate this half-foreign organism without shutting down its ability to fight infections.21PubMed Central. Tolerance of the fetus by the maternal immune system: role of inflammatory mediators at the feto-maternal interface
The solution involves overlapping mechanisms at the interface between the placenta and the uterine lining. Trophoblast cells on the outer layer of the placenta actively suppress local immune responses, and the mother’s endometrial tissues develop specialized tolerance pathways. The balance between immune memory, which protects both mother and fetus from infections, and immune tolerance, which prevents the mother from rejecting the fetus, is one of the more delicate negotiations in mammalian biology.22PubMed. Immunological memory and tolerance at the maternal-fetal interface: Implications for reproductive management of mares
The evolution of viviparity, live birth, has happened independently many times across vertebrates. In lizards, there are species where some populations lay eggs and others in the same species give live birth, with transitional forms of pregnancy occurring in between. Only three lizard species are known to display all three reproductive forms within a single species, making them invaluable for understanding how the shift from egg-laying to live birth actually happens.23PubMed Central. Understanding the evolution of viviparity using intraspecific variation in reproductive mode and transitional forms of pregnancy
Reproductive Parasites That Hijack Sex
Not all influences on sexual reproduction come from within the animal. Wolbachia, a bacterium that infects an estimated half or more of all insect species, manipulates host reproduction to favor its own transmission. Because Wolbachia is passed only through eggs, not sperm, it has evolved mechanisms to tilt reproduction in favor of infected females.
The most dramatic of these is cytoplasmic incompatibility: when an infected male mates with an uninfected female, the resulting embryos fail to develop. This effectively penalizes uninfected females and gives infected ones a competitive advantage. Different Wolbachia strains have different effects. In brown planthoppers, one strain increases fecundity without inducing incompatibility, while in small brown planthoppers, a different strain induces complete cytoplasmic incompatibility and also boosts reproductive output.24PubMed Central. Wolbachia-mediated reproductive manipulation in rice planthoppers The bacterium essentially rewires its host’s reproduction to serve its own evolutionary interests, an invisible hand shaping the sexual outcomes of billions of insects.
Environmental Chemicals and Disrupted Reproduction
Human activity is introducing a new set of pressures on animal sexual reproduction. Endocrine disrupting chemicals, synthetic compounds that interfere with hormonal signaling, are now found in virtually every environment on Earth. These substances can mimic or block sex hormones, and a growing body of evidence links them to reproductive disorders in fish, amphibians, mammals, reptiles, and invertebrates.25Advances in Experimental Medicine and Biology. Reproductive Impact of Environmental Chemicals on Animals
The effects are often subtle and hard to pin down. In sheep exposed to mixtures of environmental pollutants, researchers found changes in the structure and gene expression of ovaries, testes, and brain regions controlling reproduction, yet these changes did not always translate into measurable drops in fertility under the conditions tested.26PubMed Central. Effects of environmental pollutants on the reproduction and welfare of ruminants The concern is that individual chemical exposures may be low enough to seem harmless, but cocktails of multiple pollutants acting together could push animals past tipping points, particularly during vulnerable developmental windows like fetal development. Extrapolating from one species to another, or even from one breed to another, has proven difficult, which makes the problem both harder to study and harder to regulate.
Reproduce Once or Many Times
One last dimension of sexual reproduction worth noting is how many times an animal gets to do it. Some species are semelparous, reproducing in a single explosive bout and then dying. Pacific salmon are the iconic example, pouring every last calorie into one spawning run. Others are iteroparous, breeding across multiple seasons. Most mammals, birds, and longer-lived fish fall into this category.
The boundary between these two strategies is blurrier than it first appears. Crab spiders that normally produce a single brood have been observed laying a second one when food is unusually abundant. A small proportion of Chinook salmon, typically a die-after-spawning species, have been found surviving to reproduce in two or three additional seasons. And burying beetles that are well-fed enough can reproduce more than once, while smaller individuals breed only once. These examples place semelparity and iteroparity on a continuum rather than in strict categories, with individual condition and environmental quality pushing animals along the spectrum.